Increasing numbers of applications are being developed in the biomedical field, which involve the use of a mechanical construct that is surgically implanted to allow the body to mend or be reconstructed. These applications include spinal constructs and disk replacements, plates used for long bone repair from the femur to the metacarpals, and even soft tissue repair such as bladder and hernia repair. There are even orthopedic or dental areas in which a screw is implanted by itself to hold bone in a certain position, or to provide an anchor for a dental prosthetic.
One field which is ready for product growth is small bone surgery, i.e. below the elbow and ankle. Much of the work that has been done in this area in the past represents scaled down constructs and instruments from the large bone area. However, many of these products are not adequate for the fine bones and delicate procedures required of the small bone surgeon. These bones are fine and have minimal surface area for placement of an implant, and less mass for the placement of a screw. In addition, there is usually minimal soft tissue to cover an implant. These factors tend to make small bone surgery particularly tedious and unforgiving. Small bone surgeons are particularly appreciative of surgical tools that provide assistance in holding, centering, and implanting the delicate screws that are used in this area of the body, especially when it means that fewer hands need to be involved in the procedure.
The present invention is designed to meet the specific needs of the small bone surgeon to facilitate effective and repeatable procedures which provide for ease of use for this specific area of specialization, although it is certainly envisioned that the screw and screw driver of the present invention could be used in a wide variety of other orthopedic and surgical applications including spinal surgery, plating in various areas of the body, soft tissue repair, and bone anchoring.
The present invention provides a screw for use alone or as part of a construct which could include a plate. The screw is designed specifically for the small bone market, i.e. for use in bones distil to the elbow and knee, including, for example, the ulna, radius, tibia, fibula, as well as the metacarpals, carpals, metatarsals, tarsals, and phalanges. The screw can be used in applications previously mentioned, for example those that require fixation within a single bone such as the stabilization of a fracture or the screw can be used across two or more bones so as to facilitate total or partial fusion.
The screws are advantageously, self-starting, self-tapping screws having an internal torque driving recess that terminates in a cylindrical opening or cannulation. The cannulation can extend through the length of the screw or can extend only partially through the length of the screw in a partial cannula. The invention further includes a screw driver having an external torque driving shape and terminating in a conical or cylindrical pilot or boss which mates with the cylindrical cannula or tapered opening in the screw. The pilot is sized and held to a tolerance so as to hold the screw on the screw driver by a friction fit and further acts to center the screw's torque driving recess relative to the torque driver of the screw driver in order to help avoid stripping the torque driving recess of the driver. This is a particular problem for the small screws and even smaller torque driving recesses of the screws.
The screws include an insertion end having multiple flutes, and preferably 2 or 3 flutes about a conical recess. The screws further include a modified cancellous thread. The screw further has a partial taper of the minor diameter of about 5° to about 15°, and more preferably about 6° to about 10°, and most preferably about 8° over about the first 2 to about 6, and more preferably about 4 complete turns of the threads adjacent to the head of the screw.
The screws further include a torque driving recess that may be a hexagon, a sinusoidal shape, or a modification of a sinusoidal (multilobed) shape. This recess can be of a constant size in the direction of the longitudinal axis, or can taper inward along the longitudinal axis of the screw toward the bottom of the recess. The opening for the pilot of the screw driver can be of any cross section which corresponds to the cross-section of the pilot, but circular is preferable with a taper along the longitudinal axis in one of either the pilot or the hole. Thus, the pilot may have a slight taper and form an interference fit with a cylindrical opening, or alternatively and perhaps of slightly less advantage, the opening may have a slight taper and form an interference fit with a cylindrical pilot on the screw driver. A cylindrical opening with a slightly tapered, i.e. about 2-5°, pilot with a flat bottom works well, and is relatively easy to manufacture in the small sizes required. The driver and opening are dimensioned so that the distil end of the pilot is slightly smaller than the distil end of the opening, and the frictional fit optimally occurs right below a transition between the torque driving recess and the opening. An additional feature of the recess/opening is a chamfered transition area, which could be a truncated frutoconical shape that leads from the torque driving recess into the cylindrical opening. In addition, the head of the screw can include a rounded exterior portion or spherical shaped head to permit multiaxial insertion, i.e. in a corresponding rounded or spherical recess in a countersunk screw hole in a plate or other construct. The screws can be provided in typical lengths for small bone use, i.e. from about 5 mm to about 25 mm and typically in lengths of 8, 12 16 and 20 mm with a major diameter of about 2.7 mm or 3.5 mm. The screws can include a constant thread pitch as shown, in particular for use with a bone plate. A further embodiment of the screw for use in fixation by itself is a screw which includes a compression thread which increases in the number of turns over a given length. This variable pitch will preferably be used for the thread over about half of the distil end of the screw. The screws can be made of appropriate biocompatible material, including for example surgical grade stainless steel and titanium. The driver may be an integral design, such as stainless steel, or the pilot may be constructed as an insert for example from a more flexible material. Alternatively, the pilot may be constructed as a separate and preferably retractable assembly, such as a stylet, which rejects using the same mechanism as a retractable ball point pen. Preferably, the driver includes a fillet, or rounded transition between the shaft and the torque driving tip and also one between the torque driving tip and the pilot.